1. Field of the Invention
The present invention relates to an aqueous titanium oxide-dispersed sol, a titanium oxide film formed on a substrate of a ceramic, a synthetic resin or the like from said sol, specific titanium oxide particles, and a process for preparing an aqueous titanium oxide-dispersed sol. The titanium oxide film of the present invention is transparent and is excellent in photocatalytic activity and adhesion to a substrate.
2. Description of the Related Art
It is known that titanium dioxide (hereinafter simply referred to as “titanium oxide”) has three crystal phases, the anatase, the brookite and the rutile type. When titanium oxide is formed by combustion of titanium tetrachloride with oxygen in a vapor phase deposition process, anatase-type titanium oxide is formed and is stable at the lowest temperature. When the thus formed anatase-type titanium oxide is heat treated and the temperature is raised, brookite-type titanium oxide is formed at a temperature of 816 to 1040° C. and rutile-type titanium oxide is formed at a temperature higher than 1040° C.
Concerning a liquid process, the crystal phases of titanium oxide formed by hydrolysis of titanium tetrachloride are reported in detail by Kouemon Funaki in “Kogyo Kagaku (Industrial Chemistry)” Vol. 59, No. 11, pl295. This report concluded that rutile-type titanium oxide is formed mainly from a high concentration solution and anatase-type titanium oxide is formed from a low concentration solution. It was reported that formation of fine brookite-type titanium oxide particles in an liquid phase process was impossible.
As seen from the above, it was difficult to stably produce brookite-type titanium oxide in a liquid phase process. If the titanium oxide formed in a heated process is further heat treated at a high temperature, brookite-type titanium oxide may be obtained but the obtained titanium oxide particles have been grown by the heat treatment. Therefore, it was difficult to obtain fine brookite-type titanium oxide crystal particles.
As for the process for forming a titanium oxide sol, it is generally true that crystalline or amorphous titanium oxide particles are dispersed in a dispersing medium, or a titanium oxide precursor such as methane alkoxide, titanium sulfate or titanium tetrachloride is mixed into a dispersing medium, followed by neutralization or hydrolysis of the precursor, to form a titanium oxide sol.
A titanium oxide sol is used for producing titanium oxide particles or for forming a titanium oxide film by coating the sol on a glass or ceramic.
It is known that a titanium oxide sol is a photosemiconductor and has a transparency and an increased photocatalytic activity when its particle size is small. The photocatalytic activity of titanium oxide has recently been investigated thoroughly. The applications of the photocatalytic activity include removing harmful materials for cleaning, removing odor such as ammonia for deodorization, and sterilization of microorganisms. Titanium oxide is used in various forms such as a bulk, particles, a film and a sol, depending on the types of the applications. When the photocatalytic activity is to be combined with the transparency, the titanium oxide is often formed as a film. Accordingly, the titanium oxide is often used in the form of a sol for forming a film.
It is recognized that the photocatalytic activity of titanium oxide is higher in the rutile-type than in the anatase-type. The reason is a difference of energy gap of about 0.2 eV between the two types, as the energy gap of the rutile-type is 3.02 eV and that of the anatase is 3.23 eV (see Ceramics 31 (1996), No. 10, p 817). Because of this energy gap, the anatase-type titanium oxide is preferably used as a photosemiconductor.
As of the brookite-type titanium oxide, a pure material of brookite-type titanium oxide has not been obtained, and it was difficult to obtain fine particles of brookite-type titanium oxide having such a high specific surface area that they can be used as a photosemiconductor since the particles of brookite-type titanium oxide are prepared at such a high temperature that they are sintered.
It has been proposed that when a titanium oxide film is formed on an illuminator, for example, a glass tube of a fluorescent lump or a cover thereof, by coating it with a titanium oxide sol, organic materials such as oil smoke, when adhered thereto, are decomposed by the photocatalytic activity of the titanium oxide.
However, the sols produced by the processes described before rarely provide a titanium oxide film having a high transparency and an illuminator having a brookite-type titanium oxide film as the photocatalyst has not been known.
When a titanium oxide film is used as the photocatalyst by forming it on a glass, plastic or other substrate, it is required that the titanium oxide film has a high photocatalytic activity. Since the photocatalyst action is a reaction on the surface of particles, the particles should be fine particles having a high specific surface area and have an excellent crystillinity to obtain a high photocatalytic activity. It is also required that the film is transparent when the film is applied to an illuminator. To improve the transparency, it is desired that the particles are fine and monodispersed, as in the case of improving photocatalytic activity. Conventionally, the anatase-type titanium oxide is used and is made fine to solve the above problems.
It is also required that the titanium oxide film have a high adhesivity and peeling of the titanium oxide film should be prevented when it is formed on a substrate.
In the conventional process of hydrolyzing titanium tetrachloride, it was difficult to obtain a titanium oxide sol having a very small particle size and an excellent crystallinity of the titanium oxide particles in the sol and providing a high transparency when formed into a film.
In the process of hydrolyzing titanium alkoxide, the particles of the obtained titanium oxide sol are excellent in powder characteristics including very fine particle size, but the sol includes alcohol, which involves a safety problem that explosion may be caused when the sol is heated to form a titanium film. To prevent the explosion, a large scale apparatus for preventing the explosion is required and it is economically disadvantageous. Further, titanium alkoxide is much more expensive than titanium tetrachloride.
The object of the present invention is to provide a titanium oxide sol which can provide, on a substrate, a titanium oxide film excellent in photocatalytic activity and transparency as well as adhesion to the substrate, and to provide fine brookite-type titanium oxide particles.